chore: officially migrate to submodule (#4502)

* remove apps/core and apps/fern

* fix precommit

* add submodule updates in workflows

* submodule

* remove core tests

* update core revision

* Add submodules: true to all GitHub workflows

- Ensure all workflows can access git submodules
- Add submodules support to deployment, test, and CI workflows
- Fix YAML syntax issues in workflow files

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* remove core-lint

* upgrade core with latest main of oss

---------

Co-authored-by: Claude <noreply@anthropic.com>

examples/docs/agent_basic.py

@@ -1,49 +0,0 @@
-from letta_client import CreateBlock, Letta, MessageCreate
-
-"""
-Make sure you run the Letta server before running this example.
-```
-letta server
-```
-"""
-
-client = Letta(base_url="http://localhost:8283")
-
-# create a new agent
-agent_state = client.agents.create(
-    memory_blocks=[
-        CreateBlock(
-            label="human",
-            value="Name: Sarah",
-        ),
-    ],
-    # set automatic defaults for LLM/embedding config
-    model="openai/gpt-4o-mini",
-    embedding="openai/text-embedding-3-small",
-)
-print(f"Created agent with name {agent_state.name} and unique ID {agent_state.id}")
-
-# Message an agent
-response = client.agents.messages.create(
-    agent_id=agent_state.id,
-    messages=[
-        MessageCreate(
-            role="user",
-            content="hello",
-        )
-    ],
-)
-print("Usage", response.usage)
-print("Agent messages", response.messages)
-
-# list all agents
-agents = client.agents.list()
-
-# get the agent by ID
-agent_state = client.agents.retrieve(agent_id=agent_state.id)
-
-# get the agent by name
-agent_state = client.agents.list(name=agent_state.name)[0]
-
-# delete an agent
-client.agents.delete(agent_id=agent_state.id)